It is generally understood that any anti-skid vehicle brake system must meet the following three primary conditions, among others: provide lockup-free braking, exhibit minimal braking distance, and avoid yawing moments and steering moments during braking.
In the anti-skid control system as shown and disclosed in West German Preliminary Published Patent Application No. DE-OS 24 37 066, there is provided a device which is assigned to each brakable wheel of an axle for sensing the dynamic behavior of the wheel. The output signals from the devices are fed to evaluation circuits, which produce the output signals for controlling the braking pressure to the brakes of the left and right wheels. The evaluation circuits are interconnected to each other in such a way that, in the case of different friction coefficients (.mu.-values) for the left and right wheels of an axle (split friction coefficients), the braking pressure of the wheel running at the higher friction coefficient (high wheel) is controlled by the output signals of the evaluation circuit of the wheel running with the lower friction coefficient (low wheel). The evaluation circuits are connected to pulse devices for gradual buildup of pressure. The pulse devices interconnected to the evaluation circuits in such a way that in each case a pulse device can be switched on by the output signals of the evaluation circuit of the other wheel or by the supply valve control signal of the other wheel. For example, if the indicating signals show that only one of the wheels has a tendency to lock up, or if only one supply valve of the axis is energized, the braking pressure on the other, not-yet-controlled, wheel is built up in pulses. The pulse device then produces a fixed sequence of pulses independently of the course of the control cycles of the low wheel.
Now it can happen that at certain points in time the .mu.-valve employed for the low wheel becomes lower and that the slippage on the controlling wheel thus becomes smaller but that the high wheel that is cocontrolled undergoes a change to higher .mu. and .lambda.-values. In previous anti-skid protection equipment there is the danger of sudden instability of travel due to phase-opposed changes in the braking pressure which can cause a great steering moment on the wheels of the steering axle. Thus, there is a shock on the steering, since at the time the yawing moment increases sharply (see FIGS. 3A and 3B).